Vulnerability Note VU#118892

Older SSH clients do not allow users to disable X11 forwarding

Overview

This vulnerability may allow an attacker to make unauthorized connections to affected client machines.

Description

Older versions of the SSH client do not allow the user to disable X11 forwarding. As a result, if the client connects to a malicious server, the server can open an X11 connection to the client's display without notifying the client. The attacker would then have access to the client's X11 session, which would permit activities such as keystroke monitoring.

It is also possible for an attacker to make X11 connections to the client by taking over the victim's TCP connection; however, this should generate a client-side warning about changed host keys. If the client responds to the prompt by breaking the connection, no keystrokes or sensitive data will be transmitted.